You can stay on light duty in perpetuity. The issue is, can your employer provide light duty forever. As a work injury lawyer my answer my short answer is no. The way things work in Ohio is that if your workers comp doctor certifies you light duty IF you employer gives you a light duty WRITTEN job offer that accommodates your work restrictions you must take it.
Within the frame work of the workers comp system there is this legal concept called Maximum Medical Improvement. For the purposes of most injured workers this term comes up when he or she is off of work and receiving temporary total disability compensation. Once you are found to be MMI for the allowed conditions in your claim your compensation stops.
Now if you are on light duty and you are found MMI for your allowed conditions, then your employer would not have to pay you compensation if they did not accommodate your work restrictions. Now will they instantly pull the rug out under you, probably not, because then they may infringe upon a ADA discrimination type system. In regards to a disability discrimination type framework, the employer only has to provide a reasonable accommodation, and the employee must be able to do the primary functions of the job with or without an accommodation. Basically employer does not have to make extraordinary efforts just to ensure you still have a job.